1. Field of the Invention
The present invention relates to a catalyst composition for preparing olefin polymers, and more particularly to a catalyst composition for preparing cycloolefin copolymers with a high cycloolefin conversion and a high glass transition temperature. The catalyst composition can still maintain relatively high activity at high temperature reaction conditions.
2. Description of the Prior Art
Olefin-based polymers have been used in a wide range of applications. One group of commonly used olefin-based polymers is polyolefins, that is, homopolymers or copolymers of olefins. These polyolefin polymers are typically used in such applications as blow and injection molding, extrusion coating, film and sheeting, pipe, wire and cable.
An example of polyolefin is ethylene-propylene elastomer (ethylene-propylene rubbers, EPR). It has many end-use applications due to its resistance to weather, good heat aging properties and its ability to be compounded with large quantities of fillers and plasticizers. Typical automotive uses are radiator and heater hoses, vacuum tubing, weather stripping and sponge doorseals. Typical industrial uses are sponge parts, gaskets and seals.
Another group of commonly used olefin-based polymers is cycloolefin copolymers (COC). One of the examples is a copolymer of cycloolefin and ethylene, which has an extraordinarily high glass transition temperature compared with traditional polyolefins owing to its incorporation of cyclic monomers. Also, the polymer has high transparency in physical properties due to reduced crystallinity by the incorporation of cyclic monomers. The combination of light transparency, heat resistance, aging resistance, chemical resistance, solvent resistance, and low dielectric constant makes COC a valuable material that has attracted research activities in both academic and industrial sectors. Currently, ethylene/cycloolefin copolymers have been demonstrated to be a suitable material in the field of optical materials such as optical memory disks and optical fibers.
Ethylene/cycloolefin copolymers are usually prepared in the presence of metallocene/aluminoxane catalyst systems, as described in U.S. Pat. No. 5,559,199 (Abe et al.) and U.S. Pat. No. 5,602,219 (Aulbach et al.) In U.S. Pat. No. 5,559,199, metallocenes such as isopropylidene (cyclopentadienylmethylcyclopentadienyl)zirconium dichloride are disclosed. In U.S. Pat. No. 5,602,219, metallocenes such as dimethylsilyl-(1-indenyl)-cyclopentadienylzirconium dichloride are disclosed.
However, conventional processes for preparing ethylene-cycloolefin copolymers have some common problems. First, the conversion of the cycloolefin (or the incorporation of the cycloolefin) is too low. Second, the high incorporation of ethylene results in too low a glass transition temperature (Tg) of the copolymer.
To increase the conversion of the cycloolefin, a common technique is to increase the reaction temperature or reducing reaction pressure of ethylene. However, using this technique, the reactivity for the production of cycloolefin polymer will be reduced as the examples show in U.S. Pat. Nos. 5,602,219 and 5,559,199. Obviously, this technique will reduce the commercial feasibility for COC polymerization. Therefore, efforts to enhance the reactivity of catalyst for increasing the incorporation of cyclic olefins during the COC polymerization processes are highly desirable in the industrial applications.